ITS TIME FOR OUR SEMI ANNUAL ANALOG EXERCISE, THIS ONE FOR THE
UPCOMING WINTER, OF MATCHING CURRENT AUTUMN TEMPERATURES, ENSO
STATES AND ENSUING WINTER WEATHER RESULTS IN THE PAST. AFTER A
STRONG EL NINO LAST WINTER WHICH BROUGHT WITH IT A THOROUGH RECORD
SMASHING 78.7 INCHES OF SNOW TO PHILADELPHIA, THE TROPICAL PACIFIC
HAS TURNED THE TABLES FOR THE UPCOMING WINTER.
THE LATEST WEEKLY READINGS FROM THE TROPICAL PACIFIC HAVE THE ENSO
3.4 REGION TEETERING BETWEEN A STRONG AND MODERATE LA NINA. MOST
DYNAMICAL AND STATISTICAL MODELS ARE FORECASTING A MODERATE OR
STRONG LA NINA TO PERSIST THROUGH THE FIRST HALF OF THE WINTER
SEASON WITH VARYING DEGREES OF WEAKENING AS WE MOVE TOWARD THE
SPRING. THE LAST LA NINA WINTER OF SIMILAR STRENGTH OCCURRED DURING
THE WINTER OF 2007-8.
MODERATE TO STRONG LA NINA WINTERS UNLIKE THEIR EL NINO COUNTERPARTS
USUALLY HAVE LESS SNOW AND ALSO EXHIBIT LESS OF A CHANCE OF HAVING
LARGE SINGLE SNOWFALL EVENTS IN PHILADELPHIA. SINCE 1950 NO MODERATE
OR STRONG LA NINA WINTER (13 SEASONS IN ALL) HAS HAD A SINGLE
SNOWFALL EVENT OF 10 INCHES OR MORE AS MEASURED OFFICIALLY IN
PHILADELPHIA WHILE MODERATE TO STRONG EL NINOS HAVE HAD SEVEN (14
SEASONS IN ALL). PART OF THE CAUSE IS THE WEAKER SUBTROPICAL JET
DURING LA NINA WINTERS. THIS HAS A TWO FOLD EFFECT, ONE BRINGING
LESS TROPICAL MOISTURE THIS FAR NORTH AND TWO HAVE A TENDENCY TO
PHASE EITHER TOO EARLY OR LATE TO MAXIMIZE SNOWFALL POTENTIAL IN
OUR AREA. WE HAVE ALREADY SEEN OVER-PHASED MODELING SOLUTIONS IN THE
LONG TERM NOT VERIFYING TOO WELL THIS AUTUMN. MODERATE TO STRONG LA
NINA WINTERS OFTEN HAVE A NEGATIVE PACIFIC DECADAL OSCILLATION
SIGNAL WHICH MEANS THE WARMEST WATERS RELATIVE TO NORMAL ARE IN THE
WESTERN PACIFIC OCEAN. THIS SET-UP FAVORS RIDGING OVER THE CENTRAL
OR EASTERN PACIFIC OCEAN VS WESTERN NORTH AMERICA. THIS MEANS THE
ARCTIC INTRUSIONS ENTER THE CENTRAL TO WESTERN UNITED STATES FIRST.
THIS SCENARIO JUST OCCURRED. WHILE NOT BLOWTORCH WINTERS,
MODERATE TO STRONG LA NINAS TEND TO FAVOR SLIGHTLY MILDER THAN
NORMAL WINTERS. SINCE THE WINTER OF 1949-50, MODERATE LA NINA
WINTERS HAVE AVERAGED 35.3F IN PHL, WHILE STRONG LA NINA WINTERS
HAVE AVERAGED 35.8F. THE CURRENT WINTER NORMAL IS 34.8F.
THE RANDOM ELEMENT IN OUR WINTERS IS ALWAYS THE NORTH ATLANTIC. AS
LAST WINTER SHOWED, A STRONGLY NEGATIVE NORTH ATLANTIC OSCILLATION
CAN EVEN HOLD A STRONG EL NINO AT BAY. UNFORTUNATELY FORECASTING SKILL
OF THIS FEATURE IS NOT EASY, ESPECIALLY MORE THAN A FEW WEEKS IN
ADVANCE. LAST WINTER THE STRATOSPHERIC WINDS OVER THE EQUATOR AS
MEASURED BY THE QUASI BIANNUAL OSCILLATION (QBO) WERE NEGATIVE.
FORMER NWS METEOROLOGIST WES JUNKER FORWARDED US SOME RESEARCH
ARTICLES, WHEN THE QBO IS NEGATIVE, THE TENDENCY FOR BLOCKING AROUND
THE POLES (AND THUS THE STORMIER, COLDER AND MORE NEGATIVE NAO)
INCREASES. THE OPPOSITE IS TRUE WHEN IT IS POSITIVE AS SHOULD BE THE
CASE THIS WINTER. OTHER RESEARCH MEASURES INCLUDING THE NEWFOUNDLAND
WARM POOL (ESTIMATING A NEAR NEUTRAL NAO THIS WINTER) AND A CENTRAL
TROPICAL PACIFIC SURFACE PRESSURE/NAO CORRELATION (COURTESY OF A
MIDWESTERN COLLEAGUE, ESTIMATING A STRONGLY POSITIVE NAO) ARE NOT
SUGGESTING A NEGATIVE NAO. LAST WINTER WAS A RECORD BREAKING
NEGATIVE AO/NAO WINTER, BUT STRONGLY NEGATIVE NAO WINTERS DO NOT OFTEN
OCCUR IN PAIRS. ABOUT EIGHTY PERCENT OF THE TIME, THE SIGN SWITCHES.
STILL SOMEONE SHOULD TELL MOTHER NATURE THIS AS WE ARE ABOUT TO HAVE
OUR FOURTEENTH CONSECUTIVE MONTH OF NEGATIVE NAO(S) WITH NO END IN
SIGHT BASED ON THE LATEST MEDIUM RANGE ENSEMBLES.
WHICH FINALLY BRINGS US TO THE OCTOBER AND NOVEMBER TEMPEARTURES THIS
AUTUMN. BOTH THIS PAST OCTOBER AND CURRENT NOVEMBER WILL BE IN THE
WARMEST TERCILE (THIRD) OF ALL OCTOBERS AND NOVEMBERS SINCE 1872.
THIS WILL BE THE 23RD TIME THAT THIS HAS OCCURRED. WHEN OCTOBER HAS
BEEN WARM, 75 PERCENT OF THE ENSUING WINTERS HAVE BEEN WARMER THAN
THE CURRENT NORMAL OF 34.8F. WHEN BOTH OCTOBER AND NOVEMBER HAVE
BEEN WARM, THAT PERCENTAGE EDGES UPWARD TO 77 PERCENT. IF WE LOOK AT
JUST LA NINA WINTERS REGARDLESS OF STRENGTH, THE PERCENTAGE IS 71,
NT WHILE DURING MODERATE OR STRONG LA NINAS ITS 75 PERCENT.
SO WE HAVE SCOURED THE RECORDS TO FIND SIMILAR OCTOBER AND NOVEMBER
TEMPERATURES IN PHL DURING LA NINA FALLS AND POSTED BELOW IS WHAT
OCCURRED IN THE PAST DURING ENSUING WINTERS. WE HAVE FOUND FIVE, TWO
STRONG LA NINAS, TWO MODERATE LA NINAS AND ONE WEAK LA NINA. TWO
OTHER WEAK LA NINAS DID NOT HAVE THE SAME PDO SIGN AS IS CURRENTLY
OCCURRING IN THE PACIFIC. WITH ALL OF THOSE 70S SHOWING AS
PERCENTAGES ABOVE MAYBE IT IS NOT IRONIC THAT MANY OF THESE ANALOGS
ARE FROM THE 70S.
THIS IS BASED ON THE 138 YEARS OF RECORD IN PHILADELPHIA. THE
NORMALS ARE THE CURRENT 1971-2000 NORMALS.
YEAR ENSUING ENSUING ENSUING AVG ENSUING ENSUING
DEC JAN FEB TEMP PCPN SNOW
1950-1 34.2 36.0 36.4 35.5 9.19 4.6
1970-1 35.8 27.8 36.1 33.2 10.83 18.3
1971-2 41.5 35.1 32.4 36.3 8.64 12.2
1973-4 38.6 35.9 31.7 35.4 11.43 20.8
1975-6* 36.9 28.7 40.9 35.5 10.95 17.5
AVERAGE 37.4 32.7 35.5 35.2 10.21 14.7
NORMAL 37.4 32.3 34.8 34.8 9.57 19.3
* BEST MATCH OF ALL THE TELECONNECTION INDICES, BUT NO TWO
WINTERS ARE EVER ALIKE.
THE OFFICIAL CPC OUTLOOK FOR THIS WINTER HAS THE PHILADELPHIA AREA
AS EQUAL CHANCES OF EBING EITHER WARMER OR COLDER THAN NORMAL AND
DRIER OR COOLER THAN NORMAL.
AS ALWAYS PAST PERFORMANCE IS NOT AN INDICATION OF FUTURE TRENDS. WE
HOPE EVERYBODY HAS A VERY HAPPY AND HEALTHY UPCOMING HOLIDAY AND
WINTER SEASON. WE WENT HEAVY ON THE SALT FOR THIS WINTER, NOT EXACTLY
GOOD FOR ONE'S BLOOD PRESSURE IN MORE WAYS THAN ONE.